An organic light emitting diode (OLED), as a current type light emitting device, has been increasingly applied to a high-performance active matrix organic light emitting display. A traditional passive matrix organic light emitting display requires a shorter driving time of a single pixel as its display size increases, and thus requires increasing the transient current, which causes an increase of power consumption. At the same time, application of a large current would cause excessive voltage drop of an indium tin oxide metal oxide line and make the operating voltage of OLED too high, thereby reducing its efficiency. The active matrix organic light emitting display (AMOLED) scans progressively through switching transistors to input the OLED current, which can solve these problems well.
In the pixel circuit design of AMOLED, the major problem to be solved is the luminance non-uniformity of an OLED device driven by respective AMOLED pixel driving units.
First, AMOLED adopts thin film transistors (TFT) to construct a pixel driving unit to supply a corresponding driving current to the light emitting device. As known in the art, low temperature poly silicon thin film transistors or oxide thin film transistors are mostly used. Compared with a general amorphous-silicon thin film transistor, the low temperature poly silicon thin film transistor and the oxide thin film transistor have a higher mobility and a more stable characteristic, and are more suitably applicable to AMOLED display. However, due to limitation of crystallization technique, the low temperature poly silicon thin film transistor manufactured on a large-size glass substrate always has non-uniformity in electrical parameters such as threshold voltage, mobility and so on. Such non-uniformity would be converted into differences in driving current and luminance of the OLED device and sensed by human eyes, i.e., phenomenon of Mura color. Although the oxide thin film transistor has a better process, as similar as the amorphous-silicon thin film transistor, the threshold voltage of the oxide thin film transistor drift under pressure and high temperature for a long time. Since display pictures are different, the threshold voltage drift of thin film transistors in respective parts of the panel is different, which would cause difference in display luminance. Such difference is always presented as an image sticking phenomenon because it is related to images previously displayed.
Since the light emitting device of OLED is a current-driven device, in the pixel driving unit that drives the light emitting device to emit light, the threshold characteristic of its driving transistor has a great impact on the driving current and the final displayed luminance. The driving transistor would make its threshold voltage drift when being under voltage stress and being illuminated. Such threshold voltage drift will be reflected as luminance non-uniformity in display effect.
In addition, in order to eliminate influence caused by threshold voltage difference of the driving transistor, the design of the configuration of the pixel circuit in the pixel circuit of the existing AMOLED is generally more complex, which directly results in a decrease of production yield of the pixel circuit of AMOLED.
Therefore, in order to solve the above problem, the present disclosure has an urgent need for providing a pixel driving unit and a driving method thereof, and a pixel circuit.